A Quickhammer (sometimes seen as QH) is a technique (usually) used by scum to secure a lynch by hammering a player before anyone can react.
These are most notorious for winning the game in LyLo situations. If one Townie votes another Townie, theoretically all of the living scum can pile onto the wagon en masse and secure the game-winning lynch. However, this can be done in other situations as well in a move to cut off the Town's discussion as the game moves to Night after the lynch.
However, quickhammers that require multiple scum to participate (called an extended quickhammer) require a degree of coordination that may not be possible based on the scum players' schedules outside the game. Thus, it is not necessarily the case that a lack of an extended quickhammer implies that the person being voted is scum.
In games where it is absolutely clear that there are no barriers to scum winning via quickhammer, if a player does NOT quickhammer someone at L-1 in a LyLo situation, that player is generally considered confirmed Town. On the flip side of the coin, it is considered bad form to NOT quickhammer as scum in the same scenario - the equivalent of toying with Townies who are on the brink of losing.
If you are town, and in a position in which a quickhammer on town would win scum the game, it is a good idea to quickly post "Not hammering." or some similar phrase in order to prove that you had the opportunity and yet chose not to take it (it's important you get this post in before any potential unvotes). This confirms that either you are town or the player at L-1 is scum, and prevents players subsequently arguing that maybe you were scum who simply wasn't online.