water from washing rice, to boil thick, as gruel, rinse & comb hair
泔 centers on water used in domestic tasks: the starchy water left from washing rice, boiling something into a thick gruel-like consistency, or rinsing and combing hair with water.
泔 combines 氵 (water) with 甘 (sweet), but the exact historical development is uncertain. It may have originally referred to the sweetish water from washing rice.
Water 氵 and sweet 甘 together suggest sweetened water. Picture rinsing rice and getting cloudy sweetish water, or boiling it down into a thick sweet gruel, and even using that water to rinse and comb hair smooth.
For カン, imagine a can of rice-washing water: can -> カン, and the water inside is used for rinsing and boiling.