But I'mma purchase when that day is jerking

from Kendrick Lamar (Ft. Anna Wise & Jay Rock) – Money Trees Lyrics on Rap Genius

Meaning

When that day is troubling him or “jerkin'” (i.e. the sad memories get to him), he’s gonna buy material objects in hope that they might ease the pain momentarily.

Material objects won’t heal him, but when he’s paid (now!) he’ll cop it anyway. It’s part of the victory of his perseverance.

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