From the second edition (1989):
(ˈpɛntək(ə)l) [In med.L. pentaculum, app. f. penta- five + -culum, dim. or instrumental suffix, but actual history obscure. It. had pentacolo ‘any thing or table of five corners’ (Florio), F. had (16th c.) pentacle, something used in necromancy (Godef. says ‘a five-branched candlestick’).
As applied to something worn round the neck as an amulet, some would connect it with F. pentacol, pendacol (14th c. in Godef.) a jewel or ornament hung round the neck, f. pend- hang, à to, col, cou neck.]

A certain figure (or a material object, e.g. something folded or interlaced, of that shape) used as a symbol, esp. in magic; app. properly the same as pentagram; but also used for various other magical symbols, esp. the hexagram or six-pointed star formed by two interlaced triangles. (See also pentangle 1.)
The pentacle of Solomon, in H. More 1664, is the same as the pentangle of Solomon of Sir Gawayne c1340, Sir Thomas Browne 1646, and others.

1594 Chapman Shadow Nt., Hymnus in Cynthiam Wks. (1875) 16/2 Then in thy clear and icy pentacle, Now execute a magic miracle. 1607 Dekker Wh. of Babylon Wks. 1873 II. 200 Take Periapts, Pentacles, and potent Charmes To coniure downe foule fiends. 1616 B. Jonson Devil an Ass i. ii, They haue‥Their rauens wings, their lights, and pentacles, With characters; I ha' seene all these. 1664 H. More Myst. Iniq. i. xviii. §3 Their Pentacles which they hang about their necks when they conjure (which they forsooth‥call the Pentacles of Solomon) are adorned and fortified with such transcriptions out of holy Scripture. [1668–70 M. Casaubon Credulity & Incred. (1672) 71 By certain pentacula, and seals and characters to fence themselves and to make themselves invisible against all kinds of arms and musquet bullets.] 1808 Scott Marm. iii. xx, His shoes were marked with cross and spell; Upon his breast a pentacle. 1862 Lytton Str. Story l, You observe two triangles interlaced and inserted in a circle? The Pentacle in short. 1885 Sat. Rev. 19 Sept. 380/2 The sacramental [charm] bore a figure that looked like a rough copy of the pentacle.

Hence penˈtacular a., of, pertaining to, or of the nature of a pentacle.

In mod. Dicts.