The term 'Guitar Adjunct' refers to two items that are often used in conjunction with a guitar: an amp (3 letters) and a CAPO (4 letters). An amp is an electronic device that increases the volume of the guitar's sound, while a CAPO is a device that is attached to the guitar's neck and is used to raise the pitch of the strings. Both of these items are essential for many guitarists, as they allow them to customize their sound and create unique musical effects.