What you say when you see a hot girl/guy who you'd "put in your harem" - implies superficial attraction but not wanting to go for long term.
Obviously a reference to the Arabic حرم 'ḥaram' 'forbidden place; sacrosanct, sanctum': a place where female members of a polygamous wealthy family would reside. And be fucked at the husband's leisure. Similar to a 'seraglio'.
Suh: 'damn i'd harem that'
Kate: (at the sight of 25yo+ male) 'haaaarrreeeeeem!'
Baz: hey guys this is my sister
Theo: wtf man! no way!