Come off it is a blunt way of telling somebody not to be ridiculous. Oh, come off it, come off it, you know better.
It is friendly between friends and rude to a stranger, and the tone does all the work. It is a British shortening of an older American phrase telling somebody to come off the grass.
1phrasecome off itA blunt way of telling somebody not to be ridiculous.
“Come off it, you can't expect me to believe the dog ate your homework!”
come off it is a retort.
A British shortening of the older American come off the grass.
Originally a British shortening of "come off the grass!", an older (originally American) phrase. Come off the grass!, which is roughly indicative of the speaker's disbelief, or that the speaker believes that the one being spoken to needs to face reality, is ultimately a play on the oft-seen phrase on signs in places such as public parks: "keep off the grass".
Etymology from Wiktionary, CC BY-SA
Practised in Small Talk & Everyday Phrases