When you've had a late night out getting drunk but you wake up early with seemingly a ton of energy. So you get up and go about your day only to suddenly feel a huge energy drop, normally after you have eaten.
It's essentially a late hang-over.
(Pronounced False-E or False-E's)
Tom: "I was out really late last night with Dan and Gary but I woke up super early and I feel brilliant"
Dan: "Nah, mate. Don't fall for it. You've got the falsee's!"
....2 hours later
Tom: "I'm feeling so drained, I need to sleep so bad."
Dan: "Told ya it was a falsee!"