Yad: yood, dalet
Smol: Sin, mem, alef, lamed.

Our colleague's answer is perfectly right -- as far as what is meant by 'left hand' is the opposite of 'right hand'.
But if you mean that one hand is busy and therefore only one out of two is 'left' for other works, then we speak of
יד חופשית
yad chofshit (yadh Hophshi:th, yad hofshit, jad chofschith)
Spelling (from right to left!): yud, dalet + Het, waw, pe, shin, yud, taw.
Lit. "free hand", as opposed to a/the busy one.