The word Upanishad has been derived from the root ‘sad’, to which are added two prefixes: ‘Upa’ and ‘ni.’
The prefix ‘upa’ denotes nearness, and ‘ni’, totality. The root ‘sad’ means to loosen, to attain, and to annihilate.
Thus the etymological meaning of the word is the Knowledge or Vidya, which, when received from a competent teacher, loosens totally the bondage of the world, or surely enables the pupil to attain (i.e. realize) the Atman, or Self, or completely destroys ignorance, which is responsible for the deluding appearance of the Infinite Self as the finite embodied creature. Though the word primarily signifies knowledge.
The root ‘sad’ with the prefix ‘upa’ also connotes the humility with which the pupil should approach the teacher.
The knowledge of Brahman was considered a profound secret and was itself sometimes given the name of Upanishad.
It is to be noted that the instructions regarding Brahman were often given in short formulas also known as Upanishads. “The secret name of Brahman is Satyasya Satyam, “the Truth of truth.”
Therefore we have the instruction about Brahman in secret words like – Neti, Neti – “not this, not this.”
The Brahman is called Tadvana, the adorable of all; It should be worshipped by the name of Tadvana.” The books which contained the above mentioned secret teachings and formulas were also called Upanishads.
Source - The Principal Upanishads by Swami Nikhilananda - General Introduction Page 23
Swami Nikhilananda on Swadharma