A man that a woman is not currently dating, usually either an ex-boyfriend, a former fuck buddy or a one-night stand, that she still keeps in her phone in case of a sudden need for emergency sex.
A reserve soldier is different from a fuck buddy
or friends with benefits
in that these are people she sees outside the bedroom and are usually utilized on a more consistent basis, whereas reserve soldiers are men that she definitely does not want to date, be friends with or see for any other reason than just sex, and are usually are only called upon by a woman in cases where she's not been getting any from the guys she actually wants to date or fuck buddy is unavailable over an extended period of time and sexual frustration is on overload.
See also booty call
After her fuck buddy got a girlfriend and a series of failed dates, Megan finally gave up and called one of her reserve soldiers to quel her sexual frustration.