Toska
Etymology
From Russian тоска, related to older Slavic words for "tightness," "constriction," or "oppression" — the sense of the chest being squeezed. Vladimir Nabokov made the word famous in English in his commentary on Eugene Onegin, arguing that no single English word captures its full range from spiritual anguish down to mild boredom. His gloss is often cited as proof that the word is "untranslatable," though that reputation owes as much to his eloquence as to any special property of the word itself.
noun · /tahs-KAH/
A deep spiritual ache or restless yearning, ranging from vague soul-weariness to sharp anguish with no nameable cause.
What it names
The heavy, longing dissatisfaction that has no object you can point to — you want something desperately, but not anything in particular.
Example
By the third grey week of November a familiar toska had settled over him, a longing he could not attach to anything he actually lacked.
Related idea
Melancholy is a settled, almost comfortable sadness; toska carries a sharper edge of yearning, as if reaching for something that was never there.