"rectangular groove or channel cut out of the edge of a board or piece of stone so that it will join by overlapping with the next piece, similarly cut," late 14c., rabet, from Old French rabat "a recess in a wall, a lower section," literally "a beating down or back," a back-formation from rabattre "to beat down, beat back" (see rebate (v.); the noun is a doublet of this word).
The verb, meaning "to groove or fit (boards, etc.) by cutting rabbets" is attested from mid-15c. (implied in rabatted, rabetynge); Middle English also had rabet "joiner's plane, rabbet-plane" (mid-15c., from Old French rabot).
In Old French (and Italian), re- often appeared as ra- by confusion with the true ra- (from Latin re- + ad-), and the following consonant often was doubled; compare rabbet (a doublet of rebate), rappel (a doublet of repeal). But as ra- was not recognized in English as a prefix, words with ra- in Old French usually returned to re- in English; Rally and rabbet never were because the accent had receded. In later borrowings (rappel, rapprochement, etc.) the words tend to keep their French forms.
Intransitive meaning "pull together hastily, recover order, revive, rouse" is from 1660s. Related: Rallied; rallying. Rallying-point "place at or about which persons come together for action" is by 1798. Rally round the flag (1862) is a line from popular American Civil War song "Battle Cry of Freedom."