ungentlemanlike

Now, sir, I beg to tell you, that if your ungentlemanlike and underhand plot had succeeded, you dared not darken with your shadow the door-sill of the house you purchased.

From Charles O'Malley, The Irish Dragoon, Volume 2 by Charles James Lever

Anything which differs from this they are apt to regard as "ungentlemanlike or unladylike," and shun accordingly.

From Lippincott's Magazine of Popular Literature and Science Volume 11, No. 23, February, 1873 by Various

Although Clifford was esteemed a man of wit and a scholar, his style is rude, coarse, and ungentlemanlike, and the criticism is chiefly verbal.

From The Dramatic Works of John Dryden, Volume 1 With a Life of the Author by George Saintsbury

For ungentlemanlike conduct in his letter to Gen. Wooster, of the 25th of January last, charging your petitioner with a falsehood, and in a private manner, which is justly chargeable on himself.

From Colonel John Brown, of Pittsfield, Massachusetts, the Brave Accuser of Benedict Arnold by Archibald Murray Howe

He could have written probably eloquently and seriously enough, but to "speak like a book" would have lowered him, in his own esteem, as being unmanly no less than ungentlemanlike.

From M. or N. "Similia similibus curantur." by G.J. Whyte-Melville