Source : Webster's Revised Unabridged Dictionary (1913)

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.