To be frozen in place; to stand stunned or petrified (with fear, shock, or amazement).
夜道で突然大きな物音がして、彼は立ちすくんだ。
A loud noise suddenly rang out on the street at night, and he froze in place.
その光景を見て思わず立ちすくんでしまった。
I unintentionally stood there, stunned, at that sight.
スピーチで彼女は突然言葉に詰まり、聴衆の前で立ちすくんだ。
During her speech she suddenly stumbled over her words and froze in front of the audience.