[f. TAIL v.1 + -ING2.] That tails.

1

1899.  Buxton, in 19th Cent., Jan., 121. There is the ‘tailing’ fish [trout], feeding on caddis snail or shrimp, breaking the surface.

2

1908.  Edin. Rev., April, 391. Offering the ‘tailing’ fish a floating fly.

3