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Webster 1913 Edition

Adj.

Capable of being trained or educated;

as, boys

trainable

to virtue

Richardson.

Adj.

That may be trained. [Little used.]

trainable ‎(comparative more trainable, superlative most trainable)

trainable ‎(plural trainables)

(euphemistic, dated) A mentally retarded person who can be trained or taught.
- 1971, Harriet Eleanor Blodgett, Mentally Retarded Children: What Parents and Others Should Know:
  Toilet accidents for new children in the school are generally at least two or three times as frequent for trainables as they are for educables.