5-year-old Isabella Barrett was leaving a spray tanning sesh in NYC -- again, she's 5 -- when she went off on 3-year-old Paisley Dickey for allegedly copying her spray tan ways.

Barrett (presumably with her mother's coaching) told TMZ ... "Last time I was here, Paisley was trying to copy me ... but MY mom doesn't dress me up as a hooker."

Barrett (and her mother) are referring to the infamous "Toddlers" episode ... when Paisley dressed up like Julia Roberts as a prostitute in "Pretty Woman" for a beauty pageant.

But Barrett's insult raises some questions -- namely, how the hell does a 5-year-old know the word "hooker"?

Based on how Isabella looks to her mother for approval throughout the video, you'd think her mom coached her to say it ... but that's ridiculous, right?