This is a term used to when going to bed with someone. To make hay means to copulate in bed with a significant other, often not only for sex but for sleep too.
To take advantage of a brief opportunity while it is still there. To not waste time.
The saying comes from farming communities. There would often be only a short time in which the hay would be ready to cut- reap too soon, and the hay would be too green. Reap too late, and you run the risk of rain ruining it all. Still very good advice.
I just got a good job for the summer- I don't know if I'll have one this fall, but I'll make hay while the sun shines and save up for the winter.
To take an opportunity to do something when the time and conditions are near perfect or available.
From agriculture, when farmers gather the long grass growth of summer to dry it to produce hay as winter feed for animals; The best time to cut and dry the hay is during sunny days.
A:"There's a sale at K-mart this week - You wanna see if the have something for your sister's birthday?"
B:" Sure - Might as well make hay while the sun shines, and get her something at cut price!"
Someone who jumps from one relationship immediately into another one.
Serial monogamists can not stand to be alone and often suffer from vast commitment and insecurity issues.
Because they jump into relationships immediately after the previous one has ended, serial monogamists typically don't take the time to reflect on their behavior or why their previous relationships failed; thus, they end up making the same relationship mistakes over and over again.
Person 1: Damn, Dustin already has a new girlfriend?! It's only been two weeks since he broke up with his fiance! I think he's a sociopath.