親指
おやゆび · oyayubi · JLPT N2 · noun
thumb
親指 = parent + finger: the thumb is the parent finger, the main one of the five
親 (oya) means parent or origin. The thumb is the largest of the five fingers and the one the others are measured against, so it is the parent finger.
指 — hand radical 扌 + 旨 (zhǐ, phonetic)
指 is a semantic-phonetic compound. The hand radical 扌 gives the meaning — something done with the hand, namely pointing with a finger. 旨 supplies the sound 'zhǐ'. From 'finger' it naturally extended to the verb 'to point' and figurative senses like 'to indicate' or 'to refer to'.