Tanafur al-huruf is a burdensome or awkward combination of sounds within one word, making articulation unusually difficult. It is primarily a phonetic property rather than a judgment about the word's meaning.
An unfamiliar word can be hard because its meaning is unknown—that points toward gharaba. A word can instead be intelligible but cumbersome to pronounce—that is the domain of tanafur.
Successive articulatory gestures and clashes of sounds may contribute, but there is no valid rule that sounds from neighboring articulation points are always defective.