One of the mysteries of the English language finally explained.
Designating a method of natural birth control in which observations of body temperature and other physical indications (especially changes in vaginal mucus) are used to assess the timing of the fertile phase of a woman's menstrual cycle.
1960s; earliest use found in Union of Love. From sympto- + thermal, apparently as alteration of earlier symptothermic, itself apparently after from French sympto-thermique.
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