FLAG

to sag or droop, to become spiritless, to decline

/droop, sink, or settle from or as if from pressure or loss of tautness

SYN - droop, sag, swag

"After such fervent start his enthusiasm began to flag."

"The death of their pastor, caused the church to flag."

"The weight of these extra people caused the hammock to flag."

"As soon as she left both is spirit and his penis flagged."