-in'

G‐dropped pronunciation of -ing with /n/ instead of /ŋ/. It is inherited from Middle English -ynge, -inge, -yng, -ing, from two sources: * the Old English present participle suffix -ende; * the Old English verbal noun suffix -ung/-ing. The two Old English suffixes became confused in Middle English; although the exact mechanism of this confusion is disputed, -in' is probably derived from -ung/-ing with loss of /ɡ/ under low stress rather than from -ende.

来源：wiktionary