I first came across this word when reading Neil Gaiman’s Neverwhere. Which I’ll most likely do a review of (If you’ve never read Neil Gaiman, give his work a try, you never know you might like it!)
It simply means to be full of sadness or sorrow, very sad especially in an exaggerated or insincere way.
First known use: 1585
Some synonyms include: Mournful, Sepulchral and Melancholic.
His patriarchy often seemed lugubrious ; he would often have tears in his eyes when elucidating all my failings.
— Richard Elman, Namedropping: Mostly Literary Memoirs
Oh yes, he says, and his lugubrious expression suggests that the loss afflicts him still.
— Mary Riddell, New Statesman , September 19, 1997