Adelphi

prefix meaning "not, without," from Greek a-, an- "not" (the "alpha privative"), from PIE root *ne- "not" (source also of English un-).

In words from Greek, such as abysmal, adamant, amethyst; also partly nativized as a prefix of negation (asexual, amoral, agnostic). The ancient alpha privatum, denoting want or absence.

Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."

popular name of a diverse group of marine mammals, also including the porpoise (but the true dolphin has a longer and more slender snout), mid-14c., dolfin, from Old French daulphin, from Medieval Latin dolfinus, from Latin delphinus "dolphin," from Greek delphis (genitive delphinos) "dolphin," related to delphys "womb," perhaps via notion of the animal bearing live young, or from its shape. Popularly applied to the dorado from late 16c. through some confusion. The constellation is so called from early 15c.