To be frenzy locked is to be in the stance Frenzy without an available cancel stance. Because Frenzy causes you to take double damage, most players who use it also bring a cancel stance. If the cancel stance is not recharged, or the player has insufficient adrenaline or energy to activate it, and the player goes into Frenzy, they are said to be locked into it because they cannot escape the state of taking double damage.
Players who are Frenzy locked are especially vulnerable to spikes. If one becomes Frenzy locked, one should let their team know so that their monk can prot them.
See Cancel stance for more details.