AM and PM are associated with 12-hour timekeeping, as opposed to the international 24-hour clock. The 12-hour system grew up mainly in Northern Europe, and spread globally from there via the British Empire.
The adjective STRENUOUS has 2 senses: 1. characterized by or performed with much energy or force. 2. taxing to the utmost; testing powers of endurance. Familiarity information: STRENUOUS used as an adjective is rare.
Engage in vigorous combat or make a strenuous effort, using all one's resources. For example, I'm going to fight tooth and nail for that promotion. This expression, with its allusion to biting and scratching, was first recorded in 1576.