Could imp. : of Can

Could imp. : Was, should be, or would be, able, capable, or susceptible. Used as an auxiliary, in the past tense or in the conditional present.

Example:
How Could I test which paper towel brand is the strongest?

Taxonomy
Slang, Dictionary, auxiliary verb, cunnan, probability, Origin, OE, Usage note, word origin, kuh, 1st person, Dictionary of Idioms, Pronunciation, kæn, power failure, cough, imperative, infinitive.

Defination from the web:
Could - Definition of Could at Dictionary.com a free online dictionary with pronunciation, synonyms, and translation of Could. Word of the Day and Crossword Puzzles. ... (used to express possibility): I wonder who that could be at the door. That couldn't be true. 3. ...