dilute, thin, weak (tea), pampas grass
薄 centers on lack of density or intensity: something thin, diluted, or weak. The pampas grass sense is a specific plant whose name uses this character, likely from its thin, wispy appearance.
薄 combines 艹 (grass) with 溥, which likely contributes the sound and the idea of spreading or thinness. The character originally referred to thin vegetation or sparse grass, extending to general thinness and dilution.
Grass 艹 on top of 溥, which looks like water 氵 spreading thinly over a field 田 with a hand 寸 measuring it. Picture sparse, thin grass growing where water barely covers the ground.
For ハク, imagine a hawk flying over thin, wispy pampas grass: hawk -> ハク, and the grass is so thin the hawk can see right through it.
thin (of an object)
to fade; to become dim
thinnish; somewhat thin; weakish; on the weak side; rather light; rather pale
shortage of stock; scarcity of goods; being in short supply
slightly; lightly; thinly; faintly; dimly
thin (air); weak (liquid); dilute; sparse (population); rarefied
thinning hair; sparse hair (as in balding)
very thin; flimsy; cheap; shallow
Chinese silvergrass (Miscanthus sinensis); maiden grass; susuki grass
mint (e.g. peppermint, spearmint, etc.)
thin (type); slim; flat
being lightly dressed
dim; gloomy
filthy; dirty (looking); drab
frivolous; flippant; superficial; shallow; trifling; insincere
thinly; slightly; vaguely; a little
shorthanded; short of hands; undermanned
thin slice; slicing thin
to become weak
thin (paper, cloth, porcelain, etc.); of thin make; light (e.g. coat)