At the time you are hooking up with someone you have an extreme sensation to drop a duece you then do it right there from their belly button down to penis or vagina. Which then it is no longer a happy trail it is a grumpy trail. (this may work best when your partner is handcuffed so then they can not stop or prevent the grumpy trail)
"Yeah i guess Tyler and Mary Jo doing their casual daily routine (afternoon delight) and tyler gave Mary Jo a grumpy trail. And to top it off Mary jo is Tylers mom (RUMP)"
"It was a warm summer day at Beaver Lake, Brookes (45) and Emma (14) just got done jet skiing on the shithole they call their lake. Emma laid Brookes down on the sand then giving him a warm sensational grumpy trail. So sational Brookes moaned, "I LOVE YOU PATTY!!"