Argthtjtr

I am an English teacher. While teaching my students l am often asked about English idioms and their etymologies and meanings.
As a rule, I can find the answers to their questions.
But there's an idiom which meaning, but not the etymology, l know:
'Husband's tea'.
It means 'very weak tea'.
At my English classes I often ask the students about their associations with this idiom.
Nearly always they say, 'it's a hot, strong tea'.
Does anybody know the etymology of this idiom?
I would really appreciate your help.

The OED entry is instructive. Whilst it confirms the expression is obsolete (though I vaguely remember having heard it in my lifetime), I think the example given from 1877 comes as close as anything to explaining the etymology. The suggestion is that if a pot of tea turns out too weak, wives consider it fit only for their husband, not for it to be drunk by themselves.

husband's tea n. (also husbands' tea) Brit. colloq. Obsolete very
weak tea. 1874 Hotten's Slang Dict. (rev. ed.) at Water-bewitched
Sometimes very weak tea is called ‘husband's tea’. 1877 Rep. &
Trans. Devonshire Assoc. Advancem. Sci., Lit. & Art 9 132 A servant
girl..calls Weak tea Husband's tea, and explains that whilst wives
think such tea good enough for their husbands, they do not take it
themselves. 1895 Hampshire Tea 17 Aug. 6/4 Piled-up platefuls of
cake and bread and butter and steaming cans of tea—not ‘husbands'
tea’—were put on the tables.

According to Green's Dictionary of Slang it's a slang expression that probably compares a husband to a lover. The expression apparently was mentioned on a late 19th century edition of Hotten’s Slang Dictionary but its usage may well date earlier.

Husband’s tea (n.) [? a husband’s inadequacy as opposed to that of a lover]
very weak tea.

• 1873 [UK] Sl. Dict.


• 1889–90 [UK] Barrère & Leland Dict. of Sl., Jargon and Cant.


• 1890–1904 [UK] Farmer & Henley Sl. and Its Analogues.

Also The following source refers to husband’s tea as weak tea, but doesn’t give details on its specific origin:

The earliest usage of "weak tea" as a pejorative beverage is 1825, in Robert Forby's Vocabulary of East Anglia, in reference to the word "lap," as in: to lap up your soup. Here, though, it's as a noun: lap being a diluted sustenance such as "thin broth or porridge; weak tea, &c." The same book applies the phrase to another, wilder one: "water bewitched," a colloquialism "used derisively for excessively diluted liquor; now chiefly, very weak tea." Years later, in an 1874 slang dictionary, "water bewitched" also had this note: "Sometimes very weak tea is called ‘husband's tea.’"

(Teasquared.com)

The following extract from
The Earthen Vessel and Christian Record & Review dated 1850 appears to use husband’s tea with the above connotation:

Mr. W. shewed it was done living principally upon potatoes and bread, and a little “husband's tea;" and that he was never stronger or better in his life than at that time.