English 'had' from PIE *keh₂p- (to seize) serves double duty as both simple past and pluperfect auxiliary, and its '-d' ending is itself a fossil — likely from PIE *dʰeh₁- (to do), making 'had' etymologically something like 'have-did.'
Past tense and past participle of 'have.'
From Old English 'haefde,' the past tense of 'habban' (to have, to hold, to possess), from Proto-Germanic *habjana (to have, to hold), from PIE *keh2p- (to seize, to grasp, to catch). The PIE root *keh2p- is extraordinarily productive: it gave Latin 'capere' (to take, to seize — source of 'capture,' 'capable,' 'accept,' 'except,' 'anticipate,' 'capacity'), German 'haben' (to have), Dutch 'hebben,' and Gothic 'haban.' The dental suffix '-d-' in 'had' is the Germanic past tense marker, itself descended from PIE *dheh1- (to do, to place — the same
The '-d' ending on 'had' is itself etymologically interesting. The Germanic dental past tense suffix (-ed, -d, -t) may come from a periphrastic construction with PIE *dʰeh₁- (to do, to place). 'I had' was originally something like 'I have-did' — a 'do'-construction that was compressed into a single suffix. This is the same