Definify.com

Webster 1913 Edition

adv.

In a treble manner; with a threefold number or quantity; triply.

Swift.

adv.

trib'ly. In a threefold number or quantity; as a good deed trebly recompensed.

trebly ‎(comparative treblier or more trebly, superlative trebliest or most trebly)

trebly ‎(not comparable)

Three times, thrice (archaic)
To three times the extent or degree; triply.
- 1883, Robert Louis Stevenson, Treasure Island, Cignet Classic (1998), page 13:
  For my part, I must do my best to save this fellow's trebly worthless life; Jim, you get me a basin.