E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One of the brothers of January, an old baron of Lombardy. When January held a family council to know whether he should marry, Placebo very wisely told him to do as he liked, for says he
A ful gret fool is eny counselour,
That servith any lord of high honour,
That dar presume, or oonës [once] thenken it.
That his counseil schuld pass his lordës wit.
Chaucer: The Marchaundes Tale, line 9, 121, etc.
To sing Placbo. To seek to please; to trim in order not to offend. The word Placbo is often used to denote vespers for the dead, from the fact that it is the first word of the first Antiphon of that Office.