Laudator Temporis Acti

Phaedrus 2.5.1-4 (tr. Ben Edwin Perry):

There is at Rome a certain tribe of harlequins who are always dashing hither and yon in a flurry of excitement, very busy without having any business, puffing hard for no reward, and doing nothing with much ado. They are a nuisance to themselves and the greatest plague to others.

est ardalionum quaedam Romae natio,
trepide concursans, occupata in otio,
gratis anhelans, multa agendo nil agens,
sibi molesta et aliis odiosissima.

"Harlequin" is a somewhat odd translation of ardalio. Lewis and Short define ardalio (or ardelio, as they spell it) as busybody, as does the Oxford Latin Dictionary. The Oxford English Dictionary, s.v. harlequin, does say that the harlequin is given to "mischievous intrigue."

Another character sketch of the ardalio, from Martial 4.78 (tr. D.R. Shackleton Bailey):

Although you have stored your sixtieth harvest and your face shines white with many a hair, you run vagabond all over Rome and there is no chair to which in your unceasing progress you do not bring a matutinal "good day." No tribune can lawfully go forth without you and neither consul lacks your attendance. Ten times a day you visit the Palace on the sacred slope and your talk is all of Sigeruses and Partheniuses. Young men may act so to be sure; but Afer, there is no uglier sight in the world than an aging busybody.

condita cum tibi sit iam sexagensima messis
    et facies multo splendeat alba pilo,
discurris tota vagus urbe, nec ulla cathedra est
    cui non mane feras irrequietus 'have';
et sine te nulli fas est prodire tribuno,        5
    nec caret officio consul uterque tuo;
et sacro decies repetis Palatia clivo
    Sigerosque meros Partheniosque sonas.
haec faciant sane iuvenes: deformius, Afer,
    omnino nihil est ardalione sene.        10

Sigerus and Parthenius (line 8) were freedman, officials at the court of the emperor Domitian. On the plurals ("people like Sigerus and Parthenius") see I. van Wageningen, "Cerdo sive de nominibus propriis Latinis appellativorum loco adhibitis," Mnemosyne 40 (1912) 147-172. Afer in Martial's poem was a name-dropper as well as a nosy-parker. In an excellent article on the busybody type, Jeannine K. Brown, "Just a Busybody? A Look at the Greco-Roman Topos of Meddling for Defining ἀλλοτριοεπίσκοπος in 1 Peter 4:15," Journal of Biblical Literature 125 (2006) 549-568, there is no mention of the word ardalio.

Lewis and Short derive ardelio from ardeo, but see Rosario Moreno Soldevila, Martial, Book IV. A Commentary (Leiden: Brill, 2006), p. 502:

Ardalio seems to come from Greek ἄρδαλος, which means 'dirty' (Erot. 56.9-11), but also 'agitating' (Hesychius lexicogr. alpha 7090 ἀρδαλωμένους· ταρασσομένους). See TGL 1910 s.v. c-d; DGE s.v.; Ernout-Meillet: 44-45. The manuscript variant ardelio must be due to false etymology (Calderinus ab ardeo = festino).

Michiel de Vaan, Etymological Dictionary of Latin and the Other Italic Languages (Leiden: Brill, 2008), doesn't seem to discuss ardalio. James Westfall Thompson, "The Origin of the Word 'Goliardi'," Studies in Philology 20.1 (January, 1923) 83-98 (at 96) derives goliardi from "gula + ardelio, plural gula + ardeliones."