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Fortunately I can guess a possible ratiocination of this etymology: 5 Houseplants You Can’t Kill by Overwatering. Because these plants won't die even if soaked, immersed, or wet, they can tolerate, forigve your over-watering. They are lenient to the wrongdoing of over-watering. Am I correct? I screenshot Yellowbridge.
涵
The semantic extension is as follows:
To be soaked in water
To have a large capacity to hold water
To have a large capacity to tolerate bullshit
To be lenient (e.g. lenient on someone for their wrongdoings)
References:
涵，水泽多也。
"涵" means much water.
——《说文(Shuowen)》，an ancient Chinese dictionary
I think, the original meaning of "涵" is many rivers or lakes. Then, "涵" has a new meaning: Infiltrate; moisturize(浸润;滋润). After that, "涵" also means "contain(包含，包容)". The last meaning of "涵" is "forgiveness and tolerance(包涵，宽容)"
How does "涵" mean from "contain" to "tolerance"?
宰相肚里能撑船。
You can even row a boat in the belly of the Prime Minister Wang Anshi.
——中国俗语/Chinese saying, it means a tolerant person like Wang Anshi.
(There is also a Chinese idiom involved: a big belly can contain more things, including others' mistakes(大肚能容). It's about a big belly monk Budai, also known as Mile Buddha(布袋和尚，大肚弥勒佛).)
A lake(涵) can hold a lot of water, so the ancient Chinese compared water to people's mistakes and tolerant people to lakes.
