"full of repose," 1670s, from repose (n.) + -ful. Earlier it meant "responsible, trustworthy" (1620s), from repose (v.2). Related: Reposefulness.
Entries linking to reposeful
repose n.
c. 1500, "act or state of rest from activity, temporary inaction, sleep," from Old French repos (11c.), a back-formation from reposer (see repose (v.1)). Meaning "state of quiet, freedom from disturbing influences" is by 1650s. As a noun, 17c. also used reposal, reposance.
-ful
word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.
It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).
repose v.2
"to put, place," mid-15c., reposen, "to put (something) back;" perhaps from re- "back, again" + pose (v.) or so formed in Middle English from Old French poser, on model of disposen "dispose" [Klein], or else from Latin repos-, infinitive stem of reponere "put back, set back, replace, restore; put away, lay out, stretch out," from re- + ponere "to put, place" (past participle positus; see position (n.)). Related: Reposed; reposing.