Half-eaten, as in an unfinished meal or put aside for later.
From Latin sēmēs-us, from sēmi- + ēsus, from edēre, to eat.
1859 Farrar Julian Home vii. pg. 86 "They're sons of gyps and that sort of thing, who feed on the semese fragments of the high table."
Having forgotten to visit the supermarket, and it being way too hot to get dressed and walk back, I found it much easier to just tell my mum, who worries I don't eat healthily, that I was cooking up 'semese dans le réfrigérateur' instead of admit it was leftovers.