adjective/neuter noun; appampi is sandhi form of appaṃ + api
Analysis. appaṃ (neuter accusative singular of appa, 'little') + api (even, also) = 'even a little', emphasizing minimal quantity.
Etymology. appa derives from Indo-European *h₂ep- 'to seize, grasp'; compare Sanskrit apa 'away, off' and Latin habēre 'to have' in the sense of what one grasps or holds—thus originally 'scanty, meager' (what slips through the fingers). The prefix a- (not) + ppa- may also underlie the form, meaning 'not-full'. The particle api 'even' joins to emphasize the smallness.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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