[2786] Ṣuwāʿ (a variation of ṣāʿ, i.e. measure) is the proper noun for the instrument, while siqāyah (watering cup) is more of a description of it (cf. al-Rāzī). It is called ‘the King’s measure’ here to impart importance as the circumstance requires so (cf. Ibn ʿĀshūr).
[2787] Understandably, this was said by the senior figure among Joseph’s (عليه السلام) soldiers, who was vested with authority (cf. Ibn ʿĀshūr).