In American informal talk, a sawbuck is a ten-dollar bill. One buck is already one dollar, so a sawbuck is not “a wooden dollar” — it is specifically ten. The older, literal sawbuck is a sawhorse: a frame you rest wood on to cut it, often with legs that cross in an X. The word is saw + buck, a copy of Dutch zaagbok. The money sense grew from a picture: the Roman numeral for ten is X, and that X looks like the sawhorse. You will hear sawbuck in older American speech, westerns, crime stories, and from people who like vintage slang. A young person paying for coffee will usually just say “ten bucks.” It is not used for other countries’ money, and it is not formal.