Many words are members of a recognisable family, of which the principal members are generally nouns, verbs (and participles), adjectives, and adverbs. So: a home, to home (homing), homely; a friend, to befriend, friendly, and so on. The family tree expands when prefixes and suffixes are used as modifiers.
Apart from members of the immediate family, there also arise parallel families, cousins if you will: friendship and friendliness are both nouns that extend the concept implicit in friend; and, in a slightly different way, so do ellipse (geometrical shape), ellipsis (a truncation, especially in speech or writing), and elliptical (the adjectival form of both ideas).
Our language has thousands of examples of this. However, not all words follow this comfortable domestic trend. Some are so ungainly in shape or sound that they are ill-adapted to the rules which generally govern the relationship between the various parts of speech. Reductio ad absurdum is a useful expression, but it is incapable of being modified into an adjective or an adverb, let alone a verb. Where grammatical flexibility is needed, the only option is to use apagoge, apagogical, apagogically. Derived from the Greek, it has exactly the same meaning as the Latin.
Generally speaking, words borrowed from other languages do not take variant forms. Words like Schadenfreude, raison d’etre, hoi polloi, glasnost, and perestroika do not readily lend themselves to variations in English. However, as some loanwords become naturalised, they spawn offspring, generally to the despair of the purists. So, liaison was, until recently, considered a foreign word and treated with the courtesy reserved for visitors. But it has stayed on, and is now treated familiarly as a verb: liaise. A similar fate has met ballet (adjective: balletic); brusque (noun: brusqueness); charlatan (parallel noun: charlatanism); chauffeur (used also as a verb); clique (adjective: cliquey); elite (adjective, noun, and parallel noun: elitism); prestige (noun and adjective, and parallel adjective: prestigious), and so on. It is interesting to consider that all of these French words were treated by Fowler as truly foreign words in the first edition of Modern English Usage.
Compound words are difficult to transmute from one form to another. Lighthouse does not readily turn into a verb or adjective; bookcase likewise. Showcase, on the other hand, is used as noun and verb (courtesy of television), but does not yet boast an adjective or adverb form.
Some words have been conjured into existence because their form suggests that they are already a member of a family. This process — back-formation — is a prolific source of new words. In such a way, the noun commentator existed, and its form suggested that it came from an imagined verb, commentate. Similarly, the noun liaison existed, generating the need for a corresponding verb, and so liaise was invented. An identical process accounts for ablute from ablution; automate from automation; choreograph and choreographer from choreography; co-ordinate and co-ordinator from co-ordination; emote from emotion; extradite from extradition, and so on. Thus the apparent child begets the imagined parent.
But some words are just themselves, all alone, admitting no variant forms and used only in a single context. These orphans have lived much longer than their relatives whose remains may be found in large dictionaries. Offing is an example, now only encountered in the phrase in the offing, meaning that the thing referred to is imminent. The offing is ‘the part of the visible sea distant from the shore or beyond the anchoring ground’. A boat lying at anchor to await a favourable tide before entering the harbour is in the offing. William Dampier said in 1703, ‘By Nine a Clock at Night we had got a pretty good Offin’ — that is, a pretty good resting point some distance from the shore.
Another orphan is fell, now used only in expressions of the type one fell blow; one fell swoop. It is unrelated to the verb fall, and unrelated to the verb fell (to cause to fall). A fell blow is a cruel blow. It derives from the Latin fello-: fierce, cruel, savage. Fell meant cruel, harsh, destructive, or spirited, doughty. It has more vigorous relatives: felon also derives from fello-. Its living relatives include feloness, felony, felonry, and felonious.
Figment is confined to a corner of the workhouse where its only companion is the imagination. Its original sense is something fashioned or made. It comes from the Latin fingere: to form or mould, which is also the root of feign, fiction, and figure. While each of these has its own family and is often seen out in company, figment has not flourished. It is an invention of the mind, and it is perfectly sensible to refer to someone’s perjurious story as a figment of the mind, a figment of desperation, a figment of the imagination, or simply a figment.
Another orphan, whose parentage is not obvious, is het, as in het up. If a plaintiff knocks back a generous offer at the start of a difficult and dubious case, their counsel is likely to get het up. Although it has the appearance of a dialect word, it is the participial adjective from heat, built on the pattern feed–fed, lead–led. So it simply means heated up. It is rarely enough heard, and then only with reference to a person’s emotional temperature. I heard it often as a child: generally, it was directed at me.
We use diminutives with children without thinking. The commonest suffix which implies smallness or youth is –let. Booklet, piglet, and droplet are obvious examples; bracelet is less obvious, since the stem bracel is now obsolete. Farmlet and leaflet are common words constructed on the same lines. Much less familiar, but no less legitimate, are fanglet (small fang), doglet, froglet, goslet (small goose), sharklet (small shark), and squirelet (small squire).
Interestingly, every branch of the nobility has its corresponding diminutive. It must have been hard, as a member of the aristocracy, to be disparaged as a dukelet or kinglet. So Florio said in Montaigne (1603): ‘So many petty-kings, and petty-petty kinglets have we now adayes’. The most unkindest cut.
(Incidentally, we use orphan nowadays to signify a person who, as Lady Bracknell would have it, has been so careless as to lose both parents. But originally an orphan meant a person who has lost one or both parents.)