1801, "given to cutting a dash" (1786), which was a colloquial expression for "acting brilliantly," from dash (n.) in the sense of "showy appearance," which is attested from 1715. The sense of "splashing" is recorded from mid-15c.
1. He was the very model of the dashing RAF pilot.
2. Two elegant Scotsmen travelling together wore dashing kilts at dinner.
3. Father Auberon's Academy Club positively heaved with dashing young men.
4. He was dashing, self-confident and flirtatious.