It's not what you think it is, not a double entendre.
"But Ikey was timid, and his hopes remained insoluble in the menstruum of his backwardness and fears. "

MENSTRUUM
Webster's 1913 Dictionary

Definition: \Men"stru*um\, n.; pl. E. {Menstruums}, L. {Menstrua}.
[L. menstruus. See {Menstruous}.]
Any substance which dissolves a solid body; a solvent.

The proper menstruum to dissolve metal. --Bacon.

All liquors are called menstruums which are used as
dissolvents, or to extract the virtues of ingredients
by infusion or decoction. --Quincy.

Note: The use is supposed to have originated in some notion
of the old chemists about the influence of the moon in
the preparation of dissolvents. --Johnson.

AHD's etymology puzzles me, in view of Webster 1913 giving
no mention of relation to endocrine fluctuation.

menstruum

SYLLABICATION: men·stru·um
PRONUNCIATION: mnstr-m
NOUN: Inflected forms: pl. men·stru·ums or men·stru·a (-str-)
A solvent, especially one used in extracting compounds from plant and animal tissues and preparing drugs.
ETYMOLOGY: Middle English, menstruation, from Medieval Latin mnstruum, sing. of Latin mnstrua, menses. See menstruate.