neuter nominative singular gerundive (future passive participle) of sikkhati
Analysis. sikkhita- (past participle, 'trained') + -bba (gerundive suffix, marking obligation); the form sikkhitabbaṁ is the neuter nominative/accusative of the obligation phrase.
Etymology. From √sikh 'to learn, train' (related to Sanskrit śikṣ-), with the gerundive suffix -bba marking what ought to be done. The root itself is of uncertain origin but may relate to the idea of straightening or instructing.
Meanings from the Digital Pāḷi Dictionary; morphology and etymology by Claude.
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