that sounds a little aggressive for me. the base verb is "두다",as Channy mentioned earlier but when it is put in sentences, the verb form changes like "둬". actually it is a short form of "두어", so we can say.. 두다 > 두어 > 둬. Generally speaking "두다" means put something somewhere literally, but in this sentence it might mean "i am forcing you to be near me(cos i love you or i wanna be with you, stuff like that). it's just my two cents.

둬 = 두다 = put, place, set,etc in English 널 곁에 둬야겠어 = i must take(set) you aside of me.

"널 곁에 둬야 겠어" = "널 곁에 두고 싶어" "널 옆에 두고싶어" "네가 내 옆에 있길 원해" "너와 함께 있고 싶어"