When someone is chewing spearmint gum, is gumming on a spearmint mint, or has just brushed their teeth with spearmint toothpaste and is in close proximity to you. Secondhand spearmint refers to the disconnect between the deliciousness of said spearmint in the former party's mouth and the sickening smell the secondhand observer experiences.
"Aw dude, what's that smell?"
"Oh sorry bro, you want some gum?"
"Naw man, I just caught your secondhand spearmint. I lost my appetite."
"So why'd you dump her?"
"I went in for the kiss and all I could get was the secondhand spearmint. I can't even look at her the same anymore. We're over."