Dictionary
Words, grammatical forms and meanings linked to the ontology.
mid-season form en · NOUN
Meanings
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(especially, uncountable) Peak form, optimal form, excellent form.
I don't know if you happen to take Old Doctor Gordon's Bile Magnesia, which when the liver is disordered gives instant relief, acting like magic and imparting an inward glow? I don't myself, my personal liver being always more or less in mid-season form, but I've seen the advertisements.
1960, P. G. Wodehouse, Jeeves in the Offing, chapter IX:
Forms