soothfast (adj.)

"truth, reality, fact," Old English soð "truth, justice, righteousness, rectitude; reality, a true situation, certainty," noun use of soð (adj.) "true, genuine, real; just, righteous," originally *sonð-, from Proto-Germanic *santhaz (source also of Old Norse sannr, Old Saxon soth, Old High German sand "true," Gothic sunja "truth"). Compare forsooth.

The group is related to Old English synn "sin" and Latin sontis "guilty" (truth is related to guilt via "being the one;" see sin (v.)), from PIE *hes-ont- "being, existence," thus "real, true" (from present participle of root *es- "to be"), also preserved in Latin sunt "they are" and German sind.

Archaic in English, it is the root of modern words for "true" in Swedish (sann) and Danish (sand). It was in common use until mid-17c. then obsolete until revived as an archaism early 19c. by Scott, etc. It was used for Latin pro- in translating compounds into Old English, such as soðtacen "prodigy," soðfylgan "prosequi."

Old English fæst "firmly fixed, steadfast, constant; secure; enclosed, watertight; strong, fortified," probably from Proto-Germanic *fastu- "firm, fast" (source also of Old Frisian fest, Old Norse fastr, Dutch vast, German fest), from PIE root *past- "firm, solid" (source of Sanskrit pastyam "dwelling place").

Meaning "rapid, quick" is from 1550s, from fast (adv.) , in which entry the attempt is made to explain how a root meaning "firm, solid" came variously to yield words for "refrain from eating" (fast (v.)) and "rapid, quick." Of colors, from 1650s; of clocks, from 1840. The sense of "living an unrestrained life, eager in pursuit of pleasure" (usually of women) is from 1746 (fast living is from 1745).

Fast buck recorded from 1947; fast food is first attested 1951. Fast lane is by 1966; the fast track originally was in horse-racing (1934), one that permits maximum speed; figurative sense by 1960s. Fast-forward is by 1948, originally of audio tape.