mid-14c. (mid-13c. in Anglo-Latin), "to take or seize (property or goods) by law," a legal term, from Old French atachier "fasten; arrest" (11c.), earlier estachier "to attach, fix; stake up, support" (Modern French attacher, also compare Italian attaccare), from a- "to" (see ad-) + a base also found in detach, perhaps from Frankish *stakon "a post, stake" or a similar Germanic word, from Proto-Germanic *stakon- "a stake," from PIE root *steg- (1) "pole, stick" (see stake (n.)).
Meaning "to fasten, affix, connect," which probably is the original sense etymologically, is attested in English from c. 1400. Related: Attached; attaching.
also re-attach, "attach anew or again," in any sense, c. 1600 originally in legalese and now obsolete in that sense; see re- "back, again" + attach (v.). The general sense of "to attach again" is by 1813 and might be a new formation. Related: Reattached; reattaching; reattachment (1570s in the legal sense).