1
general::
1. Measure up, equal, as in Their gift doesn't stack up against mine. This usage alludes to piling up one's chips at poker, and comparing them to those of the other players. [Early 1900s] 2. Make sense, seem plausible, as in Her explanation just doesn't stack up. Also see ADD UP, def. 2.
American Heritage Idioms
2
general::
Phrase(s): stack something up
to make a stack of things. (Also without the up.) • Where should I stack them up? • Please stack up these boxes., Phrase(s): stack up
[for something] to accumulate, as in stacks. • Your work is stacking up. You will have to work late to finish it. • I hate to let my work stack up. I have to do it sooner or later.
McGrawhill's American Idioms And Phrasal Verbs