The phrase "overtired baby" means that a baby is extremely tired and has not had enough rest, and it can make them fussy and have difficulty sleeping. Full definition
It's extra hard because I know she wants to go to sleep, but she's all wired from the changes in her cognition, so super-short nap... which means frustration and overtired baby at bedtime. (hintmama.com)
In the cases of overtired babies, the sheer exhaustion keeps them away from sleeping. (shrewdmommy.com)
Many times overtired baby can be very active, hyper and wanting to play. (attachfromscratch.com)