This is in the spirit of the What is a Word™/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.
If a word conforms to a special rule I call it a Waiting Word™. Use the examples below to find the rule.
$$\begin{array}{|c|c|}\hline \bbox[yellow]{\textbf{Waiting Words™}} & \bbox[yellow]{\textbf{Not Waiting Words™}}\\\hline \text{AN} & \text{A}\\\hline \text{FEEL} & \text{TOUCH}\\\hline \text{FOOTSTOOL} & \text{STOOL}\\\hline \text{GNAW} & \text{CHEW}\\\hline \text{MOM} & \text{DAD}\\\hline \text{OUTDO} & \text{OUTLAST}\\\hline \text{POP} & \text{SNAP}\\\hline \text{ROTOR} & \text{WHEEL}\\\hline \text{SEXES} & \text{GENDERS}\\\hline \text{SIS} & \text{BRO}\\\hline \text{TAINT} & \text{DEFILE}\\\hline \end{array}$$
CSV version:
Waiting Words™,Not Waiting Words™
AN,A
FEEL,TOUCH
FOOTSTOOL,STOOL
GNAW,CHEW
MOM,DAD
OUTDO,OUTLAST
POP,SNAP
ROTOR,WHEEL
SEXES,GENDERS
SIS,BRO
TAINT,DEFILE