unsinkable (adj.)

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

Middle English sinken, from Old English sincan (intransitive) "become submerged, go under, subside" (past tense sanc, past participle suncen), from Proto-Germanic *senkwan (source also of Old Saxon sinkan, Old Norse sökkva, Middle Dutch sinken, Dutch zinken, Old High German sinkan, German sinken, Gothic sigqan), from PIE root *sengw- "to sink."

The transitive use "force or drag gradually downward" (attested from late 12c.) supplanted Middle English sench (compare drink/drench) which died out 14c. The sense of "go in, penetrate" (of a blow, a weapon, etc.) is from c. 1300; by early 14c. as "make a penetrating impression on the mind." Related: Sank; sunk; sinking.

From early 14c. as "be reduced to a lower or worse state;" late 14c. as "fall or fail as from weakness or under a heavy blow." From 1590s as "decrease, be reduced in value, amount, etc." Of the sun, moon, etc., "to set," c. 1600. Of land, "dip downward gradually," by 1726.

Contrasted with swim (v.) since 14c.; the adjectival phrase sink or swim is from 1660s. To sink without a trace is World War I military jargon, translating German spurlos versenkt.