Underhand is a related term of unprincipled.
As adjectives the difference between underhand and unprincipled
is that underhand is secret; clandestine while unprincipled is lacking moral values.
As an adverb underhand
is with an underhand movement.
underhand
English
Adjective
(
en adjective)
secret; clandestine
(by extension) dishonest and sneaky; done in a secret or sly manner
thrown (etc.) with the hand brought forward and up from below
Adverb
(
en adverb)
with an underhand movement
in a sly, sneaky or secret manner
unprincipled
English
Synonyms
*licentious, immoral
Related terms
*principled
*principle
References
*Oxford English Dictionary