Albert Barnes Commentary Judges 3:16

"And Ehud made him a sword which had two edges, a cubit in length; and he girded it under his raiment upon his right thigh." — Judges 3:16 (ASV)

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).