The Collaborative International Dictionary of English v.0.48:

Tonguing \Tongu"ing\, vb. n. (Music)
   Modification of tone for a rapid staccato effect by the
   performer's tongue, in playing a wind instrument, as a flute.
   In single tonguing only one kind of stroke is used, the
   tongue articulating a rapid "t;" in double tonguing, two
   strokes, as for "t" and "k," are alternated; in triple
   tonguing, "t, k, t," etc.
   [Webster 1913 Suppl.]