"slightly open, neither open nor shut," 1718, also on a jar, on the jar, perhaps from Scottish dialectal a char "turned a little way," earlier on char (mid-15c.) "on the turn (of a door or gate)," from Middle English char "a turn," from Old English cier "a turn" (see chore). For first element see a- (1). For unusual change of ch- to j-, compare jowl.
Entries linking to ajar
"a small job or task," especially "a piece of minor domestic work of regular or frequent recurrence," 1751, American English, variant of char, from Middle English cherre "odd job," from Old English cerr, cierr "turn, change, time, occasion, affair business." Related: Chores.
Chore, a corruption of char, is an English word, still used in many parts of England, as a char-man, a char-woman; but in America, it is perhaps confined to New England. It signifies small domestic jobs of work, and its place cannot be supplied by any other single word in the language. [Noah Webster, "Dissertations on the English Language," 1789]
prefix or inseparable particle, a relic of various Germanic and Latin elements.
In words derived from Old English, it commonly represents Old English an "on, in, into" (see on (prep.)), as in alive, above, asleep, aback, abroad, afoot, ashore, ahead, abed, aside, obsolete arank "in rank and file," etc., forming adjectives and adverbs from nouns, with the notion "in, at; engaged in." In this use it is identical to a (2).
It also can represent Middle English of (prep.) "off, from," as in anew, afresh, akin, abreast. Or it can be a reduced form of the Old English past participle prefix ge-, as in aware.
Or it can be the Old English intensive a-, originally ar- (cognate with German er- and probably implying originally "motion away from"), as in abide, arise, awake, ashamed, marking a verb as momentary, a single event. Such words sometimes were refashioned in early modern English as though the prefix were Latin (accursed, allay, affright are examples).
In words from Romanic languages, often it represents reduced forms of Latin ad "to, toward; for" (see ad-), or ab "from, away, off" (see ab-); both of which by about 7c. had been reduced to a in the ancestor of Old French. In a few cases it represents Latin ex.
[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose. [OED]
The spelling with j-, attested from c. 1400, is perhaps from influence of the synonymous Old French joue, which also was in Middle English (see jaw (n.)). This word and jowl (n.2) have influenced one another in form and sense. Middle English also had a jolle (late 14c.) meaning "the head," especially that of a fish, which might be from either or both nouns.