The feudal aristocracy was not the only class that was ruined by the bourgeoisie, not the only class whose conditions of existence pined and perished in the atmosphere of modern bourgeois society. The medieval burgesses and the small peasant proprietors were the precursors of the modern bourgeoisie. In those countries which are but little developed, industrially and commercially, these two classes still vegetate side by side with the rising bourgeoisie. In countries where modern civilisation has become fully developed, a new class of petty bourgeois has been formed, fluctuating between proletariat and bourgeoisie, and ever renewing itself as a supplementary part of bourgeois society.

-Communist manifesto

No one had fought more fanatically in the June days for the salvation of property and the restoration of credit than the Parisian petty bourgeois – keepers of cafes and restaurants, marchands de vins [wine merchants], small traders, shopkeepers, handicraftsman, etc. [...] Salvation of property! But the house they lived in was not their property; the shop they kept was not their property; the commodities they dealt in were not their property. Neither their business, nor the plate they ate from, nor the bed they slept on belonged to them any longer. It was precisely from them that this property had to be saved – for the house-owner who let the house, for the banker who discounted the promissory note, for the capitalist who made the advances in cash, for the manufacturer who entrusted the sale of his commodities to these retailers, for the wholesale dealer who had credited the raw materials to these handicraftsman. Restoration of credit! But credit, having regained strength, proved itself a vigorous and jealous god; it turned the debtor who could not pay out of his four walls, together with wife and child, surrendered his sham property to capital, and threw the man himself into the debtors’ prison, which had once more reared its head threateningly over the corpses of the June insurgents.

The petty bourgeois saw with horror that by striking down the workers they had delivered themselves without resistance into the hands of their creditors

-The Class Struggles in France

There's not many places where he just has a dictionary type definition, there's many things like that however in many places in his writings.