Definition of "tonic"

Out in front and across the street, Doc noted half a dozen or so young men, not loitering or doing substances but poised and tonic, as if waiting for some standing order to take effect.

2009, Thomas Pynchon, Inherent Vice, Vintage, published 2010, page 316

GABAergic neurones in the basal ganglia output nuclei have high tonic firing rates (40-80 Hz).

Peter Redgrave (2007) Basal ganglia. Scholarpedia, 2(6):1825.

Charlie ain't no Nazi / She likes to wear her leather boots / 'Cause it's exciting for the veterans / And it's a tonic for the troops.

1978, “She's So Modern”, performed by The Boomtown Rats

'You're a tonic, Dee,' she said. 'And a real friend. Thanks.'

2011, Cathy Kelly, She's the One

The result is the perfect tonic for Newcastle, coming at the end of a week that saw the departure of Andy Carroll to Liverpool on Monday and an injury to Shola Ameobi during Wednesday's defeat at Fulham.

2011 February 5, Paul Fletcher, “Newcastle 4 - 4 Arsenal”, in BBC‎

When all signs of effusion, dulness, pain, œgophony, and cough had disappeared he was dieted, stimulated, and tonicked.

1887, Medical Press and Circular, volume 94, page 461

The Persians, as a nation, were first to discover that fish were edible. The time is fixed at about 3000 B.C. This was their secret for some centuries—until the Assyrians learned about the elegance of fish for tonicking the brain.

1939, Frank Grant Menke, Encyclopedia of Sports, page 17