While many modern dictionaries combine not oneself and not feeling like oneself under a single entry, native English speakers use them to describe two distinct conditions: behavioral state versus physical health.
Direct Definitions & Core Difference
- Not Oneself (Behavior & Temperament): Acting in an unusual, out-of-character manner, typically being irritable, moody, flustered, or distracted. It focuses on behavior. While physical illness can be the cause of this behavior, the idiom itself is used only to describe the (and usually apologize for or explain) the behavior.
- Not Feeling (Like) Oneself (Physical Health): Experiencing internal physical discomfort, illness, low energy, or malaise. It focuses on health.
Usage Nuances & Examples
1. “Not Oneself” (An Apology for Bad Demeanor)
Use not oneself (or not myself, not himself, not herself) when acknowledging an emotional shift or apologizing for an out-of-character reaction:
- “Please excuse Sarah if she seemed distant at lunch; she’s had a rough week and is just not herself.”
- “I shouldn’t have raised my voice at you during the meeting. I’m not myself today.”
2. “Not Feeling (Like) Oneself” (Describing Physical Illness)
Use not feeling oneself or not feeling like oneself when describing the onset of a cold, fatigue, or general physical discomfort:
- “I’m going to head home early; my head is pounding and I’m just not feeling like myself.”
- “He’s been lying on the couch all morning—he’s definitely not feeling himself.”
Why Dictionaries Get This Wrong (The Lexicographical Conflation)
If you look up “not oneself” in major reference works, you will often see physical illness listed right alongside behavioral changes. For instance:
- Cambridge Dictionary: Defines not yourself as “not feeling well or not acting in your usual way.”
- Dictionary.com: Defines not oneself as “not feeling physically or mentally well.”
- The Free Dictionary: Blends them together as “not feeling as one normally should, either physically, mentally, emotionally, etc.”
The Source of the Mistake
This conflation happened because lexicographers focused on causal overlap rather than conversational usage:
- Sickness Causes Bad Temperament: When a person is physically ill, they often become irritable, quiet, or snappy. Because physical illness frequently results in out-of-character behavior, dictionary editors lazily tacked “physically ill” onto the definition of not oneself.
- Ignoring Native Speaker Selection: In actual spoken English, native speakers choose these phrases based on what they are trying to communicate:
- If you are apologizing for snapping at a coworker, you say: “I’m sorry, I’m not myself today.” (Focus = behavior)
- If you are calling in sick or leaving the office early due to a fever, you say: “I’m heading home, I’m not feeling like myself.” (Focus = physical health)
By lumping both under a single entry, standard dictionaries obscure the functional distinction that native speakers intuitively make every day.
Related Expressions
- Out of Character: Behaving in a way that is atypical for a person’s usual personality.
- Out of Sorts: Feeling physically strange, cranky, unhappy, or unlike one’s usual self.
- Under the Weather: Slightly sick or indisposed.
- Off One’s Game: Performing below one’s usual standard (in sports, work, or focus).
- Beside Oneself: Overcome with intense emotion (such as grief, panic, or rage).