"stun-locked" is video game terminology, at least as far as my understanding of it goes.
Essentially some act "stuns" you, in this case a direct challenge or statement made by the guy verbally assaulting him sends him reeling. Each statement is made successively so he was stunned by each statement... hence "stun-locked" as in he was stunned, then stunned again, then again, and had no time to collect himself or respond.
In video games "stun-locking" usually means to render another player out of control of their character temporarily, but when the effects are chained the other player can do basically nothing until the perpetrator has run out of ways to keep the "stun-lock" going.
(Explanation: in Final Fight, you can normally only chain 3-4 hits on an enemy before knocking them down, which makes them invulnerable until they stand up. However, if you miss an attack, it resets the counter. Turning around, punching the air, and then landing another hit can be done in less than the length of most enemy stuns.)
Essentially some act "stuns" you, in this case a direct challenge or statement made by the guy verbally assaulting him sends him reeling. Each statement is made successively so he was stunned by each statement... hence "stun-locked" as in he was stunned, then stunned again, then again, and had no time to collect himself or respond.
In video games "stun-locking" usually means to render another player out of control of their character temporarily, but when the effects are chained the other player can do basically nothing until the perpetrator has run out of ways to keep the "stun-lock" going.