1530-40; perhaps aphetic variant of empester,impester to tangle, encumber (though pester is found earlier than these 2 words) < Middle Frenchempestrer to hobble, entangle < Vulgar Latin*impāstōriāre to hobble, equivalent to im-im-1 + pāstōri(a) a hobble, noun use of Latinpāstōrius of a herdsman or shepherd + -āre infinitive suffix (see pastor); aphetic form apparently reinforced by pest (cf. -er6)
C16: from Old French empestrer to hobble (a horse), from Vulgar Latin impāstōriāre (unattested) to use a hobble, from pāstōria (unattested) a hobble, from Latin pāstōrius relating to a herdsman, from pastor herdsman
1524, "to clog, entangle, encumber," probably aphetic of M.Fr. empestrer "place in an embarrassing situation" (Fr. empêtrer, Walloon epasturer), from V.L. *impastoriare "to hobble" (an animal), from L. im- "in" + M.L. pastoria (chorda) "rope to hobble an animal," noun use of L. pastoria, fem. of pastorius "of a herdsman," from pastor "herdsman," from pascere "to graze." Sense of "annoy, trouble" (1562) is from influence of pest (q.v.).