untrusty

But now I must tell thee, that it is for thy sake that I have been untrusty to my lord. 

From The Water of the Wondrous Isles by William Morris

I had crept like an untrusty man into the poor 243 maid’s affections; she was in my hand like any frail, innocent thing to make or mar; and what weapon of defence was left me?

From The Works of Robert Louis Stevenson - Swanston Edition Vol. 11 (of 25) by Robert Louis Stevenson

The old Prince demanded his daughter of him who had proved so untrusty a guardian.

From Aucassin & Nicolette And Other Mediaeval Romances and Legends by Eugene Mason

I had crept like an untrusty man into the poor maid's affections; she was in my hand like any frail, innocent thing to make or mar; and what weapon of defence was left me?

From David Balfour, Second Part Being Memoirs Of His Adventures At Home And Abroad, The Second Part: In Which Are Set Forth His Misfortunes Anent The Appin Murder; His Troubles With Lord Advocate Grant; Captivity On The Bass Rock; Journey Into Holland And France; And Singular Relations With James More Drummond Or Macgregor, A Son Of The Notorious Rob Roy, And His Daughter Catriona by Robert Louis Stevenson

One word of his despite in question call'd my name; Two words of his untrusty tongue brought me to open shame.

From A Select Collection of Old English Plays, Volume 6 by William Carew Hazlitt