late 14c., diafragma, in anatomy, "muscular membrane which separates the thorax from the abdominal cavity in mammals," from Late Latin diaphragma, from Greek diaphragma "partition, barrier, muscle which divides the thorax from the abdomen," from diaphrassein "to barricade," from dia "across" (see dia-) + phrassein "to fence or hedge in," which is of uncertain etymology. Beekes suggests it is a substrate word and finds "no convincing correspondence outside Greek."
The native word in the anatomical sense is midriff. From 1650s as "a partition" of any kind, "something which divides or separates;" 1660s in the special sense "thin piece of metal" serving some purpose (as a sound resonator, etc.). Meaning "contraceptive cap" is from 1933. Related: Diaphragmal; diaphragmatic.