This is the 5297th most frequent Polish word.
ręku
"Ręku" is the locative or dative form of "ręka," meaning "hand" in English.
In this sentence, 'ręku' refers to the physical hand in which the subject is holding something.
Trzymałem kwiaty w ręku.
I was holding flowers in my hand.
Here 'ręku' refers to the wrist area of the hand, where a watch is typically worn.
Na ręku miałem zegarek.
I had a watch on my wrist.
In this sentence, 'ręku' is used to specify which hand (right) is involved in the action.