Something tedious goes on and on and is dull — not hard, just long and empty of interest, so that doing it or sitting through it tires you out. That combination of LENGTH and DULLNESS is the whole point: a two-minute job is not tedious, however dull it is, and a long job you enjoy is not tedious either. It fits repetitive work perfectly — copying numbers into a spreadsheet, checking a hundred names one by one — and it fits long meetings, long speeches, and long films. It is a slightly formal, educated-sounding word; in casual speech friends more often say 'so boring' or 'such a pain'. Two things to notice about how people really use it. First, it is very often used about the small unavoidable parts of something worthwhile: 'The idea was exciting, but the paperwork was tedious.' Second, applied to a PERSON ('a tedious man', 'don't be tedious'), it is a cool, rather cutting insult meaning they go on and on — colder and more looking-down than 'boring', and mostly British in flavour.