What was “scutage” in medieval England?

The story behind the answer

In the feudal system, tenants who owed knight service could commute that duty by paying cash to the crown; this levy, assessed per “knight’s fee,” let lords avoid mustering while still meeting obligations. Kings, especially Henry II and John, prized it because cash could hire professional mercenaries and sustain campaigns beyond the short terms of feudal levies.

It wasn’t a fine for refusing summons, a tax specifically funding the royal household, or a land grant (that was a fief). Discontent over frequent, high-rate scutages under King John fueled baronial resistance and contributed to Magna Carta, which sought consent for such exactions through the “common counsel” of the realm.