Upon his right thigh - The proper side for a left-handed man. It would give him the appearance of being unarmed. The narrative shows clearly that his action was premeditated Judges 3:21.
Related Commentaries of Judges 3:16
Judges 3:16
16 But Ehud made him a dagger which had two edges, of a cubit length; and he did gird it under his raiment upon his right thigh.