Oh, you were so heavy in my heart, boo
Heavy can be interpreted as both “meaningful” and “ballast"
"Boo” can be read as both an affectionate name for a lover and a cry of remorse, like “boo-hoo”
Tying in with the previous line, Yoni is no longer weighed down by this girl and is taking off like a hot air balloon which has been set free of its sandbags. However, she still means a lot to him.
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