"secreting internally," 1914, from endo- + Latinized form of Greek krinein "to separate, distinguish" (from PIE root *krei- "to sieve," thus "discriminate, distinguish"). Denoting glands having an internal secretion.
word-forming element indicating "branch of knowledge, science," now the usual form of -logy. Originally used c. 1800 in nonce formations (commonsensology, etc.), it gained legitimacy by influence of the proper formation in geology, mythology, etc., where the -o- is a stem vowel in the previous element.
The second element is prop[erly] -logy (-logue, etc.), the -o- belonging to the preceding element; but the accent makes the apparent element in E[nglish] to be -ology, which is hence often used as an independent word. [Century Dictionary]
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Definitions of endocrinology from WordNet
the branch of medicine dealing with the endocrine glands and their secretions;