Dull, humdrum, dreary, monotonous, pedestrian, stodgy mean so unvaried and uninteresting as to provoke boredom or tedium.
Dull (see also STUPID) implies the lack of all that gives brightness, edge, or point to the person or thing; it need not imply inferiority, but it does suggest, from the point of view of one who judges, a want of interesting character.
Humdrum implies a commonplace and routine character; it suggests a lack of variation that persists and colors the life or the people who lead that life.
Dreary (see also DISMAL) applies to something that from the writer’s or speaker’s point of view seems uninteresting and dull; the word may imply an absence of enlivening character in the thing itself but more often it reveals an attitude of mind.
Monotonous implies an irksome sameness (as of what never changes in quality, character, or appearance); it may be widely applied (as to work, to play, to persons, to scenes, or to noises).
Pedestrian, which basically means walking on foot, is applied chiefly to something written, especially in verse, that lacks any quality (as originality in thought or freshness in expression or in imagery) which raises the spirits and that, therefore, is monotonous and uninspired; the term may connote a dull prosaic quality and implies the impossibility of lifting the reader’s thoughts, emotions, or imagination.
Stodgy, which also implies the lack of a quality which inspirits or inspires, is wider in its application, for it emphasizes the heaviness, the solidity, or the lumpishness of something (as a person, a book, or an affair) that should be lighter, brighter, or gayer.