This dissertation explores the multifaceted Korean interjection 'ney' from the pragmatic, multimodal, and historical perspectives. As a polite expression, 'ney' demonstrates a speaker’s awareness of the socio-cultural expectation for individuals to use honorifics appropriately, indicating their relative status vis-à-vis their interlocutors. Being employed in diverse interaction scenarios to build effective communication and cultivate interpersonal relationships, 'ney' has become one of the most frequently used expressions in contemporary Korean.Though it was initially regarded as a dialectal expression of the standard Korean expression 'yey' and/or an incorrect expression of 'nyey', 'ney' has developed into a versatile device due to the feature of flexibility and adaptability as an interjection. Eventually, 'ney' has come to be used across various interaction contexts and was finally recognized as a word of the standard Korean expressions in the late 20th century.The multifunctionality of 'ney' can be broadly classified into a response form and a discourse marker. As a response form, 'ney' conveys a speaker’s attitudinal stance of politeness and emotional stance of (dis)affiliation toward the interlocutor’s utterances or ongoing interaction. In some cases, 'ney' politely provides a speaker’s affirmation, confirmation, and acceptance, and indicates a speaker’s agreement, interest, presence, and departure, signaling affiliation. In some other cases, it politely elicits a desirable response, requests an interlocutor’s reiteration, and interrupts an interlocutor’s undesirable utterance, signaling disaffiliation. As a discourse marker, on the other hand, 'ney' is used to politely start an utterance, elaborate on a previous utterance, backtrack a previous utterance, summarize a previous utterance, and manage a topic at the utterance-initial position. It is also used to fill a pause and hold a conversation turn at the utterance-medial position and to finish an utterance and yield a conversation turn to the interlocutor at the utterance-final position. These various discourse-pragmatic functions are associated with the different acoustic features (such as pitch, break, and length) and nonverbal behaviors (such as head nod, gaze, and body orientation).The development of 'ney' from politely providing affirmation to emerging as a discourse marker for organizing discourse and negotiating a conversation turn in a polite way can be viewed as an instance of grammaticalization. From a broader perspective toward grammar, 'ney' has acquired new grammatical functions, following the general tendencies observed in the grammaticalization into discourse markers, including phonetic erosion, layering, divergence, and specialization. As 'ney' has become widely used, 'yey' has become specialized for marking formality. Additionally, as 'ney' was acknowledged as a standard language expression, its archaic form 'nyey' became an incorrect expression. This instance illustrates how a linguistic device can vary its usage by conforming to socio-cultural norms, indicating the dynamic interrelationship between language, culture, and society.