polish off vs cap off

polish off and cap off both are verbs.

Word	Noun	Adjective	Verb	Adverb
polish off	No	No	Yes	No
cap off	No	No	Yes	No

As verbs, cap off is a hyponym of polish off; that is, cap off is a word with a more specific, narrower meaning than polish off:

polish off (verb)	cap off (verb)
finish a task completely	finish or complete, as with some decisive action
finish eating all the food on one's plate or on the table
kill intentionally and with premeditation