adj. sli·er (slīər), sli·est (slīĕst) also sly·er or sly·estIdiom:
1. Clever or cunning, especially in the practice of deceit.
2. Stealthy or surreptitious: took a sly look at the letter on the table.
3. Playfully mischievous: a sly laugh.
on the sly
In a way intended to escape notice: took extra payments on the sly.
[Middle English sleigh, from Old Norse slœgr.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.