oistrum Save DMLBS oistrum, oistreum [ME, OF oistre < ostrea < ὄστρεον] , oyster (Ostrea edulis). 1338 coquina ‥ in ~is iiij d. Ac. Ep. Bath. 145; 1339 in oystreis emptis pro camera prioris j d. Ac. Trin. Dublin 1; 1454 item in ~is et in foc’ [? l. focali] iiij d. DCCant. DE/16.