Have not had any idioms or phrasal verbs for a while so thought we’d make amends with some that are amusing but can be insulting. More will follow.
‘He’s/she’s three sandwiches short of a picnic’ means, or is used to say someone is stupid, not very intelligent or that they have done something stupid. The implication being there is something missing from the normal. Because, if you had enough sandwiches for a picnic, everything is okay but if you haven’t got enough i.e. you’re missing some, it would imply the picnic will be a failure.
Likewise, ‘the lights are on but nobody’s in’ means, or is used to describe someone in a similar manner as above, that they are stupid or that they are thinking about something else therefore they do not react to something that is said to them.