Praise be to Allah.
It is soundly narrated that Ibn Mas‘ood (may Allah be pleased with him) said: May Allah curse the one who does tattoos and the one who has a tattoo done, the one who has her facial hair plucked, and those who file teeth for the purpose of beautification, changing the creation of Allah. Why should I not curse those whom the Messenger of Allah (blessings and peace of Allah be upon him) cursed, when it is in the Book of Allah? “And whatever the Messenger has given you - take; and what he has forbidden you - refrain from” [Al-Hashr 59:7].
Narrated by al-Bukhari (5931).
It was narrated that Ibn ‘Abbaas (may Allah be pleased with him) said: The woman who does hair extensions and the woman for whom that is done, the woman who plucks facial hair and the woman for whom that is done, the woman who does tattoos and the woman for whom that is done when there is no disease (that would justify doing that) are cursed. Narrated by Abu Dawood (4170); its isnaad was classed as hasan by Ibn Hajar in Fath al-Baari (10/376) and as saheeh by al-Albani in Saheeh Abi Dawood (no. 4170).
The scholars quoted these hadiths as evidence that plucking the hair from the eyebrows is not allowed.
It says in al-Mawsoo‘ah al-Fiqhiyyah al-Kuwaitiyyah (14/81):
The fuqaha’ are agreed that pulling out the hairs of the eyebrows comes under the heading of plucking facial hair that is prohibited. End quote.
Definition of the eyebrow:
The hair of the eyebrows is the hair that grows over the brow bone that is above the eye.
Ibn Manzoor (may Allah have mercy on him) said:
The word haajibayn (translated as eyebrows) refers to the brow bones above the eyes, with the flesh and hair that cover them. And it was said that the word haajib refers to the hair that grows over that bone. It is so called because it shields (yahjib) the eye from the sun’s rays."(Lisaan al-‘Arab 1/298-299).
This is what the word haajib refers to in linguistic terms and as is customarily known.
Based on that, what appears to be the case is that this hair asked about is connected to the eyebrow, because even though it is separate from the main hair of the eyebrow, it is still connected to it, as it grows in the region of the eyebrow bone, so it comes under the same ruling.
One should avoid a dubious issue regarding this matter, especially since the view of the majority of scholars is that plucking facial hair other than the eyebrows also comes under the heading of namas (plucking) that is prohibited.
And Allah knows best.