To rationalise

This verb is used of a psychological process whereby a person justifies with false reasoning an action which he or she wishes to take irrationally. In Jane Austen's Emma, the eponymous heroine wants to prevent Harriet Smith from marrying Robert Martin for selfish and meddling motives but rationalises her interference by telling herself that it is for Harriet's own good.

Trouthe

When reading Chaucer it is vital to realise that some words, which appear like modern English terms, have important differences in meaning. Trouthe does not mean truth but is a solemn promise on which a person's entire integrity resides and which must not be broken under any circumstances. The Franklin's Tale in The Canterbury Tales has its narrative base in two such conflicting promises.

MOOD and TONE

In literary analysis it is helpful to acknowledge or create a distinction between these terms:
mood represents the writer's feelings towards his/her subject matter; tone results from her/his attitude to the topic.
The mood might be sad or joyful or any other emotional state whilst the tone could be ironic, adulatory etc and is more a mental state.

DIMINUTIO

This is a rhetorical device used by speakers and by Chaucer in The Canterbury Tales whereby a character disparages his own skills, often claiming to be a blunt person, incapable of elevated discourse. This allows him to be crude or, alternatively, impressive with poetic language which comes as a surprise. The Franklin says he is a "burel" [plain, homely] man and yet tries to win admiration with a Tale about aristocrats with some rhetorical flourishes.  The device may still be heard when someone starts a speech: "Unaccustomed as I am to public speaking."