I put him up點解?

That is an example of Phrasal Verb.
Put up (verb + adverb)

It means to provide food and lodgings for a person (in your case, that is 'him')
Sample sentence: Mrs. Smith said she would put the student up only if he could find nowhere to stay.

There are at least 8 other different meanings for PUT up, but none of them are used with a pronoun (a person).
I am afraid the phrase is not a slang.

If you wish to study more about Phrasal Verbs, I suggest you getting a Phrasal verb guide book, English with Chinese explanation is a better choice. Longman is a good choice if you are still studying in school.

put somebody up:
1. provide food and accommodation for somebody
e.g. We can put you up for the night.
我們可以招待你過夜.
2. Present somebody as a candidate in an election
提名某人為候選人

I put him up :
I encourage him / I persuade him (to do something)

呢d叫俚語嗎 – yes – slangs
Look up in the internet and type “slang”, there are a few web sites with many slangs, but not all because slangs are being developed everyday.


2008-08-02 02:23:25 補充：
Hi joancowjoancow, please go to the web site : Wikditionary and look for "put up". Please let me know your opinion after you see that. Thanks.