An Etymological Dictionary of Astronomy and Astrophysics

Exactly suitable; appropriate; suited to a purpose.

Etymology (EN): M.E., from L. aptus “fitted, suitable, fastened,” from p.p. of *apere “to fasten;” akin to Hittite hap- “to attach;” PIE *ap- “to grasp, take, reach”.

Etymology (PE): Niyâw “apt, suitable, appropriate,” from Mid.Pers.