Old Irish sengainn

Howdy all,

I came across the term "sengainn" and am trying to determine the
meaning of this word. I believe it is Old Irish (although I may be
wrong)
My modern Irish dictionary had a somewhat similar term meaning "thin"
although I am unsure as to whether the two words stem from the same
root.

Any help with sengainn would be greatly appreciated.

Sincerely,
Aydan