Now, as in the Latin and other languages, a yawning fellow is synonymous or equivalent to a negligent and sluggish fellow; this idle custom ought certainly to be avoided; being (as was observed) disagreeable to the sight, offensive to the ear, and contrary also to that natural claim, which every one has, to respect. For when we indulge ourselves in this listless behaviour, we not only intimate that the company we are in does not greatly please us; but also make a discovery, not very advantageous to ourselves; I mean, that we are of a drowsy, lethargic disposition: which must render us by no means amiable or pleasing to those with whom we converse.

[Et ho io sentito molte volte dire a’ savi litterati che tanto viene a dire in latino «sbadigliante» quanto ’neghittoso’ e ’trascurato’. Vuolsi adunque fuggire questo costume, spiacevole -come io ho detto- agli occhi et all’udire et allo appetito; perciò che, usandolo, non solo facciamo segno che la compagnia con la qual dimoriamo ci sia poco a grado, ma diamo ancora alcun indicio cattivo di noi medesimi, cioè di avere addormentato animo e sonnacchioso; la qual cosa ci rende poco amabili a coloro co’ quali usiamo.]

Giovanni della Casa (1503-1556) Florentine poet, author, diplomat, bishop

Galateo: Or, A Treatise on Politeness and Delicacy of Manners [Il Galateo overo de’ costumi], ch. 3 (1558) [tr. Graves (1774)]
    (Source)

(Source (Italian)). Alternate translations:

A yawner meaneth as much in Latin as a careles and Idle bodie. Let us then flye these condicions, that loathe (as I said) the eyes, the Eares, & the Stomacke. For in using these fashions, we doe not only shewe that we take litle pleasure in the company, but we geve them occasion withall, to judge amis of us : I meane yt we have a drowsye & hevie nowle, which makes us ill wellcom, to all companies we come unto.
[tr. Peterson (1576)]

Many times have I heard learned men say that in Latin the word for yawning is the same as that for lazy and careless. It is therefore advisable to avoid this habit which, as I have said, is unpleasant to the ear, the eyes, and the appetite, because by indulging in it we show that we are not pleased with our companions, and we also give a bad impression of ourselves, that is to say, that we have a drowsy and sleepy spirit which makes us little liked by those with whom we are dealing.
[tr. Einsenbichler/Bartlett (1986)]