Toska

Toska (n): Russian

No single word in English renders all the shades of toska. At its deepest and most painful, it is a sensation of great spiritual anguish, often without any specific cause. At less morbid levels it is a dull ache of the soul, a longing with nothing to long for, a sick pining, a vague restlessness, mental throes, yearning. In particular cases it may be the desire for somebody or something specific, nostalgia, love-sickness. At the lowest level it grades into ennui, boredom.

--Vladimir Nabokov

Nodding my head vigorously.