mother-in-law
姑 centers on a woman related by marriage, specifically a mother-in-law. The other meanings (father-in-law, aunt, temporarily) are less common and may be context-dependent, but the core idea is a female in-law.
姑 combines 女 (woman) with 古, which likely contributes the sound and may relate to the idea of an older female relative. The exact historical development is uncertain.
A woman (女) who is old (古) in the family: think of the traditional mother-in-law figure.
For コ, imagine a mother-in-law who is always saying 'co-co' like a hen clucking around the house: co -> コ.
Ma Gu; legendary Taoist immortal with long talon-like fingernails
mother-in-law
underhanded; unfair
Sagittaria trifolia var. edulis (edible variant of threeleaf arrowhead)
sister-in-law
dilly-dallying and temporizing (temporising)
mother-in-law (of the husband)
Chinese water chestnut (Eleocharis dulcis var. tuberosa)
girl (esp. Chinese); young unmarried woman
(in China) 5th note of the ancient chromatic scale (approx. F sharp)
daughter-in-law and mother-in-law (relationship, conflict, etc.)
conflict between a bride and her mother-in-law; bride and mother-in-law war
timid and evasive (person); timid and makeshift (approach, measure, etc.)
Chinese water chestnut (Eleocharis dulcis)
parents-in-law
things happening exactly as one pleases (wishes); someone being very attentive to one's wishes
Eleocharis kuroguwai (species of spikerush)