指甲
zhǐ jia · HSK 5
fingernail
💅 指 (finger) + 甲 (shell/armor) = the hard shell on your finger
指 means finger and 甲 originally meant armor or hard shell (also used for turtle shells and fingernails). Together they literally name the hard plate on each 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'.
甲 — pictograph of a seed sprout's hard shell
甲 originally pictured the cracked shell of a seed as it sprouts, or the hard carapace/armor of a turtle or insect. From this came meanings like 'shell,' 'armor,' and 'helmet.' It was later borrowed as the first of the ten Heavenly Stems, giving it the sense of 'first' or 'A-grade.'