What is the meaning of “Nanny’s words made janie”s kiss across the gatepost seem like a manure pile after rain”

Nanny forbids Janie to kill any boy she does not okay.

"Whut Ah seen just now is plenty for me, honey, Ah don’t want no trashy nigger, no breath-and-britches, lak Johnny Taylor usin’ yo’ body to wipe his foots on."

She reduces Janie's kiss to "manure".

no i dont know