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Webster 1913 Edition

Noun.

The burden of a song; the chorus; the refrain.

Dryden.

Accompanying strain; subordinate and underlying meaning; accompaniment; undertone.

In the very [poetry] there often an

undersong

of sense which none beside the poetic mind . . . can comprehend.

Landor.

Noun.

Chorus; burden of a song.

Menalcas shall sustain his undersong.

undersong ‎(plural undersongs)

The burden of a song; the chorus; the refrain.
Dryden.
Accompanying strain; subordinate and underlying meaning; accompaniment; undertone.
an undersong of sense which none beside the poetic mind […] can comprehend. — Landor.