this is an abbreviation of: “to be so mother f*cking god damn lonely”
it comes from the song abbreviation ‘tbsl’ (To Be So Lonely by Harry Styles)
this slang/abbreviation is used a lot in wattpad comment sections when
people feel really lonely compared to the probably romantic
scene they are reading.
it is widely used by (mainly
directioner, harries, louies etc) wattpad users and is an inside joke from within the fandoms