What is the French phrase (or hyphenated word) for describing frivolities?

You possibly mean the English word (obviously from the French) froufrou.

froufrou noun (or frou-frou)

plural froufrous or frou-frous

• 1 a rustling especially of a woman's skirts
• 2 showy or frilly ornamentation

froufrou adjective

variants: or frou-frou

: very showy or fancy … fussy, fragile, froufrou clothes won't make it through the winter.

[Merriam-Webster]

You are probably trying to recall the French frou-frou.

Its basic meaning seems to be onomatopeoic - representing the rustle of fancy dresses as their affluent wearers walked about in the late 19th century. So the Cambridge English defines it as:

decorative pieces added to women's clothing:

The Merriam Webster offer a short article:

Nineteenth-century Europe featured a lot of sophisticated fashions—especially in Paris, a city considered by many to be the fashion capital of the world. Women's dresses were often made of drooping layers of fabric (such as satin or silk) that rustled as the women moved around, and "froufrou" was the French word coined in imitation of the sound they made. The word made its first appearance in English in 1870 as a noun meaning "rustling." It later came to mean "ostentatious decoration," and its usage expanded beyond the world of fashion to other crafts such as architecture and interior design. These days it also shows up as the adjective frou-frou, meaning "very heavily decorated and fancy," as in "frou-frou designs."