Depicts putting something sweet in the mouth (甘) with a spoon. Based on the original meaning "very peaceful and happy". The meaning later shifted to "excessive" "extremely".
Shuowen Jiezi
《說文》：“甚，尤安樂也。从甘，从匹。耦也。，古文甚。”
Original meaning
very peaceful and happy
Components
甘(iconic) — Depicts something sweet inside the mouth.