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1.
german root word "lund" meaning turd and "er" referring to the space between the scrotum and anus (also called the gooch)
Modern meaning combines the two to form a person who is ferociously dirty, obsessed with sports, and has abnormally small genitals.
Possible suffixes include -fanny -butt -gut -stench and most importantly -pit
"Yo cuz! Me and Andre was snackin on our fried chicken and grape soda this morning and we spotted this lunder comin out of the shower"

"Quit rubbing your duck butter on my towel you filthy lunder!"
by John Thompson June 21, 2004
 
2.
When a "lighting" and "thunder" occur at the exact same moment. Creating a fusion on both words.
What was that? Was it a lighting or what is a thunder? Oh it was a lunder.
by marcolalala June 19, 2014
 
3.
A meal in the late afternoon to substitute for lunch and dinner.
Lunder at 4 pm.
by sak135 March 14, 2009