Hard work especially work that makes you feel physically tired. (Cambridge Dictionary)
Work extremely hard or incessantly. (Oxford Dictionaries)
Move slowly and with difficulty. (Oxford Dictionaries)
Long strenuous fatiguing labour. (Merriam – Webster)
When people toil, they work very hard doing unpleasant or tiring tasks. (Collins Dictionary)
If you toil somewhere, you move there slowly and with difficulty, usually because you are very tired. (Collins Dictionary)
Toil: A Word Every Engineer Should Know.
The toil to discover ‘new physics’ after Higgs boson breakthrough.