Aghlich is a Middle English word believed to mean "fearsome," "awful," or "terrible." It's only known occurance is in line 136 of Sir Gawain and the Green Knight. Here are lines 134-7:
For vneþe watz þe noyce not a whyle sesed,
And þe fyrst cource in þe court kyndely serued,
Þer hales in at þe halle dor an aghlich mayster,
On þe most on þe molde on mesure hyghe.
Etymologically, the word is similar to the modern English word "ugly," which also combines the Old Norse root ag- ("to be feared") with the Old English adjectival suffix -lic, which has since become the modern English "-ly"

StrawberryFrog points out that Afrikaans (another Germanic language) has virtually the same word, "aaklig," meaning awful, terrible, or very bad.